Dr. Matthew Ellis, the study's lead author, said tumors can become resistant to the drugs over time. He said if tumors stop responding to the drugs, researchers found giving doses of estrogen kept cancer from growing in 30 percent of women.

"About a third of them will respond to receiving low doses of estrogen and then, when that stops working, in about a third of patients you can go back and use the estrogen lowering agents again."

Ellis said researchers aren't sure how many times patients could be treated with a cycle of varying estrogen levels before tumors would become resistant to it.

The research is published in the August 19 Journal of the American Medical Association.